Machines are currently available for producing espresso coffee which comprise an infusion chamber, inside which a charge of ground coffee or a single-serving sachet; in the form of a capsule, cartridge, pod or the like, containing coffee powder, is placed, and through which pressurized water is subsequently made to flow at a sufficiently high temperature to extract the flavors of the coffee powder, with which the beverage is prepared.
The characteristics of the beverage that can be obtained with these machines can vary also on the basis of the settings provided by the user. For example, these machines can produce espresso coffee at high pressure, forming the typical froth that distinguishes this beverage, or a longer coffee, such as filter or so-called fresh brew coffee, which is produced utilizing lower pressures and larger quantities of water.
Solutions suitable to select the quality of coffee obtained are currently utilized both in machines for private or domestic use and in machines for commercial or professional use, or in vending machines in which dispensing of coffee is subordinate to payment (using coins or prepaid cards) of the cost of the beverage.
EP-A-1.133.944 describes a machine for producing coffee, in which the infusion chamber is provided with a dispensing device designed to be able to produce, as chosen by the user, different qualities or types of coffee, for example more or less concentrated. More specifically, the publication EP-A-1.133.944 describes a device for producing coffee which comprises a supply line for hot water delivered from a pump, a coffee dispensing device, an infusion chamber into which the hot water supply line flows and out of which the coffee dispensing duct, which is part of the dispensing device, flows. The infusion chamber is delimited by reciprocally movable parts; one of which is in the form of a piston, which slides with respect to a cylinder. To allow different qualities of coffee to be dispensed, a valve is positioned on the dispensing device and provided with a plug element associated with a preload spring and means to allow modification of the cross section of the coffee outflow duct, to provide a first passage with a smaller cross section operating at a higher pressure and with a lower flow rate, and a second passage with a larger cross section to operate with a higher flow rate and lower pressure.
In particular, one of the embodiments described in EP-A-1.133.944 is provided with a valve to regulate the flow rate and a throttling device, controllable manually or also by means of an actuator, set by the user of the machine, to increase or decrease the cross section of the passage for the coffee. By modifying the section of the passage, a higher or lower pressure can be generated inside the infusion chamber, thereby also modifying the flow of coffee dispensed and the water delivered to the infusion chamber, obtaining different qualities of coffee, which can be selected by the user.
For similar functions and objectives, the subsequent publication EP-A-1.267.685 describes a machine for producing coffee, in which the cross section of the passage for the coffee is modified to switch from a larger cross section to a smaller cross section or vice versa, acting on operation of the pump delivering the pressurized water. More specifically, in EP-A-1.267.685 a coffee dispensing valve is provided, designed to react automatically to the delivery pressure of the water, so that a higher delivery pressure causes throttling of the valve and therefore slower outflow of the coffee. By making the pump deliver the water at a lower pressure, and therefore, for example, making the pump run at a power below the rated power, the delivery pressure of the water in the infusion chamber is below the value which causes automatic throttling of the coffee outlet valve. The flow rate of the water and hence of the coffee dispensed is higher than in the previous case and the production pressure is lower, to obtain a weaker coffee than the coffee obtained with delivery of water at high pressure.
Therefore, the machine described in EP-A-1.267.685 requires control of the pump in order to operate. Operating the pump at different pressures can cause modification of the structure of the beverage dispensing valve and coffees of different qualities can thus be obtained.